Acoustic sensors are used in underwater environments for a variety of purposes, such as geophysical exploration, depth detection and anti-submarine warfare. Because of the nature of the environment, these sensors often provide the only information on the physical conditions of the surroundings. Therefore, it is vital that the sensors be accurate and calibrated properly.
It is known in the art to use piezoelectric elements in acoustic sensors, particularly those in underwater environments. U.S. Pat. No. 4,536,862 to Sullivan discloses a sensor having piezo-electric elements attached to conductive plates. When external forces act on the sensor, the conductive plates and corresponding piezo-electric elements flex in response thereto, with the piezo-electric elements generating an electrical signal in relation to the applied force. However, Sullivan is silent on the manner in which the sensor is calibrated.
U.S. Pat. No. 5,995,451 to Evans et al. discloses an underwater sensor using piezo-electric elements and accelerometers. The reference discloses a first calibration method for the piezo-electric elements and the accelerometers that use a function generator, a speaker and a sound pressure level meter. A second calibration method places the sensor in a fluid stream with a known flow rate so that a calibration table can be produced. However, neither of those calibration methods can be implemented in a self-contained sensor or in the field.
The Sullivan reference fails to disclose a calibration method and the components that are needed to perform a calibration procedure. The Evans et al. reference discloses a calibration method, which in the first instance requires a speaker and a sound pressure level meter, and in the second instance requires the user to produce a calibration chart. The Evans et al. reference therefore requires several components for the calibration procedure, specifically the speaker and the sound pressure level meter, and it also requires that the calibration take place prior to deployment of the sensors.